


Big Bro's Lesson

by daemonhart



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Anal Sex, First Time, Incest, Loss of Virginity, M/M, Mating Cycles/In Heat, POV First Person, Plot What Plot/Porn Without Plot, Self-Lubrication, Sibling Incest
Language: English
Status: Completed
Published: 2018-02-12
Updated: 2018-02-12
Packaged: 2019-03-17 06:58:39
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,634
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13653837
Author URL: https://archiveofourown.org/users/daemonhart/pseuds/daemonhart
Summary: When Sans has trouble, he seeks advice from his big brother. Today, he has a different problem than usual.





	Big Bro's Lesson

**Author's Note:**

> Despite his behaviour, I intended for Sans to still be an adult in this, much like Papyrus is still an adult in the canon game.

“Bro, help…” I look up from my book to see my little brother standin’ before me. He shifts his weight from one foot to the other, like a human kid that’s gotta use the bathroom. It physically can’t be that, so my pupils lift to meet his, waitin’ for clarification. He continues to shuffle restlessly, and maybe it’s my imagination, but he seems even more anxious once our eyes meet. “Something’s wrong,” he whimpers. “I’m  _leaking_.” The surprise on my face must not translate well, because he immediately draws into himself with fear. “It it bad? Am I gonna die?!”

I take a second to gather the words I wanna use. Before I say anything, though, I gotta be sure. “I don’t think so, bro. How about you show me what’s goin’ on? I think I’ve got an idea, but it’s better to be sure.” His legs shift again. He’s shy. He must’ve had to save all his courage to even ask me. “It’s alright. If it’s what I think it is, it’s totally normal. Besides, I’m not gonna hurt you or pick on you. I’m your big bro. This is what I’m here for.”

His eyes cast downward, but he nods. “O-okay…Don’t make fun of me!” His gloved fingers hook into the hem of his pants and start to push them down. As they pass down his femurs, I can already see the sticky blue fluid trying to stay connected between his pelvis and the crotch of the garment. Exposing his bones to the cooler air makes him groan under his breath. The pants soon get caught around the tops of his boots, so he steps forward to free himself and leave it all in a pile. “See…? I don’t know where it’s coming from.”

The scent that hits me as his movement stirs the air is utterly indescribable. How can he be so turned on but not have any idea what to do? Maybe I really am too overprotective…”Come closer, Sans. It’s alright. I think you’ve just got a lotta magic backed up in your bones.” How many heats had gone ignored? How many times had he gone on with his day horny out of his mind but too innocent to understand? As he nears I can see it, the way his own magic condenses on the outside of his pelvis and trails along its curves to the symphysis, then gathers until it drips free like thick fluid. I’ve only read about it. I already knew how to shape my dick before I had my first heat, and even if I didn’t, dad would have  _relished_ teaching me, probably just how I was looking forward to this. “C’mere, Sansy. Sit on my lap and I’ll show you how to treat it,” I offer.

His legs tremble, but still he steps forward. He subconsciously can’t resist, but he’s got a real strong will. “But…you’ll get dirty,” he protests. “You and the couch.”

“Don’t worry about it. I can wash my shorts. Sit,” I urge again. I’m more direct, and another large drop of magic hits the carpet. He liked it. “Besides, you’re gettin’ it all over the floor right now.” He blushes once he realises, then hurries over. With my help, he gets situated comfortably in my lap, back pressed into my hoodie.

“Now what?” he asks.

“Now, you trust me. Okay?” My hand traces over the iliac crest, down along the iliac spine…He shivers, but doesn’t fight it. I can practically feel his eyes burnin’ a hole in my hand as he takes it in. As my fingers draw closer over the pubic bones, he squirms and whines.

“It feels weird! I don’t know if I like this…” he says. “I’m really warm…I think I have a fever, bro. I should go to bed…” He tries to lean forward and slide off, but my other arm wraps tight around his chest.

“No, little bro…That’s what got you in this mess. It’s not a fever,” I almost growl. I  _have_ to be patient with him, but I want to just plow into him right now. At last, I curl my index finger beneath the symphysis and rub it hard along the curve of his pubic arch. He practically squeals in delight - legs spreadin’ wide, head tossin’ back onto my shoulder, fluid leakin’ all over my phalanges. While he’s reeling, I raise my digit to my mouth and lick it clean. It’s so intensely lush with magic that I can’t help but feel my shorts start to tighten under him. I didn’t mean to conjure it, but…

He rolls his hips once his body notices the bulge, but I don’t think he realises why he did it. “Papy~! Ah…W-what was that? Can you do it again?” He turn his head to try and look at me with those adorable, big eyes of his. My precious little brother feels no shame, nor does he realise that what I’m doing is wrong. When my hand returns, I purposely reach beneath him. I gotta get him to conjure that hole…I flick the little tip of his coccyx and that earns me a sharp gasp, but it’s not enough. “Papy, why? Why’s this happening to me…?” I don’t answer right away. I pinch the small bone, then rub my thumb roughly up toward the sacrum. This time, it works. The fluid congeals and solidifies properly into a ring of muscle. I can teach him the rest later. I drag the points of my fingers over the new orifice and his voice rings out. It’s all still coated in excess magic but that’s fine with me. All the easier on both of us. I take a moment to adjust and tug at the hem of my own shorts until my cock pops free. “B-bro, what’s that?”

The arm around his chest tightens and the other slips beneath his knees to help hold him up while I get myself nice and aligned. The moment the blunt head nudges his hole, his bones shudder against one another. “Papy…? What are you doing?” I nuzzle my face alongside his. His arms curl up over mine, keeping it secure around his torso. Even like this, he’s seeking reassurance from me, believing every bit that I’ll take care of him. I’m disgusted that I’m taking advantage of him, but it’s too far to go back now. Instead, I exhale hard.

“I’m fuckin’ you.”

The wail he lets loose echoes in our living room once I pull him down onto my swollen dick. The fluid-slicked muscle gives way easily, but even still I stop him once he’s halfway down. I lift him, then pull him down again. Another cry. His fingers dig into the sleeve of my arm, but I don’t stop. I’m acutely aware that I’m using him like some big fucktoy, but what am I supposed to do? Again. Again. Sans gives a shuddering sob, and it forces me to pause. Am I hurting him? “Bro…you alright?” He shivers, but with both my arms holdin’ him up, I’ve got no way to comfort him. “I’m sorry…You gotta hate me for this…”

He sniffles, then looks back at me again. “W-why are you so mean?” he asks. I don’t know what to say. I open my mouth and no words come, so I glance away. “It hurts…Why are you  _teasing_ me, Papy?” My face tinges orange. What? “It hurts all the way inside…but you just keep rubbing a little before you take it away again…Help me, please?” My breath hisses between my teeth as I clench my jaw. Keeping him held to me, I move us both so his ribs press into the couch and his ass juts in the air, all covered in a drape made of me and my hoodie. He immediately looks more comfortable; he pulls a throw pillow close to cling to it while he arches his spine downward to present himself. “Please~”

“Mnh, that’s a good boy…Big bro will help you as much as you want!” I snap my hips forward and bury my cock inside while he mewls. The fluid squelches every time I move, giving a filthy undertone to his sweet, desperate voice. It’s smearing on my shorts, dripping to the couch, but neither of us care.

“Yes! More! More~ Oh, Papy, it’s not…it’s not enough~” he pleads, even as he drools all over the pillow. “I hurt so bad…I need something…Please, bro, please!” He starts pushing back to meet my thrusts, following the guidance of instinct to try and earn what he’s begging for. “More! Ah, more! Make me feel good~ I want it~” Between his voice and him tryin’ to ride my dick, I’m not able to last much longer. I shove both my hands onto his shoulders to pin him and let loose. Every thrust comes quick and hard like a dog doing its damnedest to get his bitch pregnant. He’s a howling, sobbing mess, but under it all I can hear his mantra of “more, more, more” creaking out in his breaking voice. Suddenly, his bones shudder hard and the ectoplasmic hole constricts tight around me. “ **Papy**! Oh, gods…!” Sure enough, he drags me over the edge right along with him. Thick pulses of semen empty into the conjured space, making it swell its thin, blue membrane to contain it all. “Ah, yeah! Oh, it feels good~ More and more…fill me up…” His heat finally begins to settle, but it’ll be a while yet before he’ll want me to pull out. Maybe he’ll ask me to go again, just to make sure I chased that bad feeling away.

I can’t say I’ll say no, either.


End file.
